1. Field of the Invention
This invention relates to a plug-hole-installed ignition coil unit for internal combustion engines, particularly to an ignition coil unit installed in a hole opened through the engine cylinder head for direct ignition which houses a circuit board on which an ionic-current detection circuit and succeeding processing circuits for misfiring detection are formed.
2. Description of the Related Art
There have recently been proposed plug-hole-installed ignition coil units which are installed in a hole opened through engine cylinder head (so-called xe2x80x9cplug holexe2x80x9d). Since such an ignition coil unit is installed in the plug hole, the coil unit is almost free from environmental influence such as moisture, oil, or any other foreign objects and can exhibit stable ignition performance. An example of the plug-hole-installed ignition coil unit is disclosed in Japanese Laid-Open Patent Application No. Sho 63 (1988)-120,863 in which a power transistor for switching operation of the primary coil is integrally housed so as to effect stable ignition performance in a reduced size.
In a gasoline or other spark-ignition internal combustion engine, a high voltage generated by the ignition coil is applied to spark plugs installed in the individual cylinders. The spark discharge that the high voltages produces across the gap between the spark plug electrodes ignites the air-fuel mixture, causing combustion. However, when certain causes are present during the engine ignition/combustion stroke, the combustion of the air-fuel mixture does not proceed normally, i.e., misfiring occurs.
Causes of misfiring fall in two classes, those attributable to the fuel supply system and those attributable to the ignition system. Misfiring attributable to the fuel supply system is the result of either excessively lean or excessively rich air-fuel mixture. In this case, a spark discharge is produced across the gap of the spark plug but the air-fuel mixture does not ignite. Misfiring attributable to the ignition system is the result of spark plug fouling (smoldering) caused by deposits of soot, etc., or of a problem in the ignition circuit that prevents normal spark discharge (mis-sparking).
When the air-fuel mixture burns normally, the combustion is accompanied by ionization of the air-fuel mixture (more precisely the combustion gas produced by normal burning of the air-fuel mixture) that gives rise to ionic-current. When misfiring occurs and the air-fuel mixture does not burn, the air-fuel mixture does not ionize and no ionic-current arises. A widely used method of detecting misfiring has therefore been to detect the ionic-current occurring during the combustion stroke using the spark plug as a probe for ionic-current detection and comparing the detected value with a prescribed value.
When conducting the misfire detection using this plug-hole-installed ignition coil unit, the ignition coil unit would normally be separated from the ionic-current detection circuit and other processing circuit and be connected by a cable or wire. That would degrade the accuracy of ionic-current detection and misfire detection. To be more specific, since the ionic-current resulting from combustion can be present for a extremely small period of time after ignition, the detection must be implemented at a fixed timing in a prescribed period of time.
However, if the ignition coil unit is connected to the circuits by a cable or wire, the detection accuracy of ionic-current and misfiring could be degraded by environmental influence. Specifically, the resistance in the cable or wire could be varied with the change in environmental temperature. The stray capacity of the wire or cable may be affected by moisture, oil or foreign conductive materials deposited thereon. The capacity in the circuit capacitors may also be varied by these factors. When these happen, it makes difficult to conduct the detection at a fixed timing in a prescribed period of time.
Moreover, when the ignition coil unit and the circuits are connected by a wire or cable, since the flowing current could rise up to 30 kV, it may become necessary to take some countermeasures against this high voltage by, for example, using a high-tension code. In addition, this high voltage current flow may generate noise in other electronic units.
A first object of the present invention is to overcome the aforesaid problems and to provide a plug-hole-installed ignition coil unit having an improved detection accuracy of ionic-current detection and misfiring.
A second object of the present invention is to provide a plug-hole-installed ignition coil unit which does no longer require to take high-voltage countermeasurement and can avoid generation of noise in other electronic units.
For achieving the first object, this invention provides a n ignition coil unit to be installed in a hole opened through a cylinder head of an engine, comprising: a case; an ignition coil installed in the case and comprising a primary coil connected to a power source and a secondary coil connected to a spark plug which is installed in a combustion chamber of a cylinder of the engine for producing spark discharge to ignite air-fuel mixture to generate combustion thereof in the combustion chamber; and a circuit board on which an ionic-current detection circuit, connected to the secondary coil, which detects ionic-current that flows during the combustion of the air-fuel mixture; and a processing circuit, connected to the ionic-current detection circuit, which generates an output indicative of the detected ionic-current, are formed; wherein the improvement comprises: the circuit board is installed in the case with the ignition coil.